1. Field of the Invention
This invention relates to an apparatus and method for testing motor vehicles under anticipated road conditions.
2. Description of the Prior Art
Test apparatus in the form of dynamometers is widely used for testing motor vehicles in place. Since the test vehicles are not moving over a road bed, the dynamometer must simulate certain forces normally associated with actual vehicle operation. These parameters include forces associated with inertia (related to the mass or weight of the vehicle) and road load forces (related to the velocity of the vehicle). The vehicle engine (or its braking system) must overcome inertial forces in order to accelerate or decelerate the vehicle. In addition, the engine must overcome breakaway frictional and rolling frictional forces (i.e., road/tire friction) as well as windage forces (i.e., drag forces caused by air passing over the vehicle). These latter forces are commonly referred to as road load (RL) forces and may be represented by the formula: EQU RL=Av+BvV+CvV.sup.2 +DW
where Av represents the vehicle constant load coefficient (e.g., effects of breakaway force), and, Bv and Cv represents the vehicle load coefficient dependent on velocity and velocity squared (e.g., windage), and D represents the grade coefficient (e.g., slope of the grade). It should be noted that a load coefficient based on velocity cubed may be added if desired. V represents the vehicle velocity and W represents the vehicle weight.
The purpose of the dynamometer is to impose those forces on the vehicle which the vehicle would incur during actual operation on a road. Chassis dynamometers for 2WD vehicles (front or rear axle drive) include a roll (or a pair of rolls) for engaging the driven wheels of the vehicle being tested. Chassis dynamometers for 4WD vehicles (front and rear axles coupled to the engine) include a roll or pair of rolls for supporting and engaging each pair of wheels (front and rear) with each pair of rolls being free to rotate independently and at different speeds or electrically (or mechanically) coupled so that all of the rolls rotate at the same speed. See U.S. Pat. No. 5,101,660 assigned to the assignee of this application.
Typically a power supplying and absorbing unit such as an electric motor (a.c. or d.c.) or a power absorbing unit such as a friction brake, eddy current brake or hydrokinetic brake is coupled to the roll or rolls for supplying power to and/or absorbing power from the roll(s) which in turn applies a force to the surface of the vehicle wheels (e.g., tires) to simulate the road load forces. Inertial forces can be simulated by power supplying and absorbing units during both acceleration and deceleration but can be simulated by power absorbing units only during acceleration. Mechanical flywheels are generally used in conjunction with power supplying and/or absorbing units to simulate a part (or in some instances substantially all) of the vehicle inertia. Vehicle velocity may be determined from the formula: ##EQU2##
where V.sub.1 =the computed velocity at time t.sub.1, V.sub.0 =the velocity at time t.sub.0, F=the measured force at the wheel/roll interface, I=the vehicle inertia to be simulated (e.g., vehicle weight in lb or Kg) and RL=the road load force and dt=the interval of integration. The implementation of this basic equation to control the operation of a dynamometer is explained in some detail in U.S. Pat. No. 4,161,116 also assigned to the assignee of this application.
The rotational velocity of the roll is representative of V and can be accurately measured by coupling a speed encoder of the optical or magnetic pulse type to the dynamometer roll. However, there is no force measuring device which, as a practical matter, can be placed between the rotating vehicle wheel and the roll. As a compromise, a force measuring device or transducer is generally placed either at the output of the power supplying and/or absorbing unit or between the flywheel assembly and the shaft connecting the flywheels to the roll. In either case, there are bearing friction and windage losses generated by the roll and/or flywheels which are not measured by the transducer. Such losses are commonly referred to as parasitic losses and must be compensated for in order to provide an accurate control signal for the power supplying and/or absorbing unit in the dynamometer.
A parasitic loss profile or curve of the lost force at the roll surface (due to parasitic losses) versus roll speed for the roll can be computed by measuring the force required to maintain the roll or rolls at several selected (e.g., four or more) speeds. A signal representative of the forces attributable to parasitic losses and the dynamometer out-of-loop inertia must then be added to the force signal measured by the transducer to provide a force signal representative of F.
The use of the above algorithm (implemented with the dynamometer parasitic losses and out-of-loop inertia) for controlling the power supplying and/or absorbing unit is compromised by the forces attributable to the inertia of the out-of-loop rotational mass of the vehicle such as the wheels, drive train etc. While the rotational mass of such components is generally small (e.g., equivalent to 50 to 70 or more Kg) compared to the weight of the vehicle (e.g., 500 to 2500 Kg) the resulting inaccuracy in the road condition test results may prove to be very expensive to a manufacturer which is required to meet maximum fuel performance requirements (as set, for example, by the U.S. Environmental Protection Agency ["EPA"]) for its vehicles.
An EPA advisory circular (A/C No. 55c dated Dec. 12, 1986) suggests that the rotational mass of the vehicle may be set at a value up to 3% of the vehicle inertia (i.e., vehicle weight) where the vehicle rotational mass is not known. This figure may be added to either the out-of-loop inertial mass of the dynamometer or to the mass of the vehicle. While such a rough approach will provide better test results than neglecting the vehicle rotational mass altogether, it does not account for the variations in rotational mass among the vehicles to be tested. For example, 4WD vehicles may have higher percentages of rotational mass than 2WD vehicles.
There is a need for a versatile chassis dynamometer apparatus and method for more accurately testing motor vehicles under simulated road conditions.